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A common margin of excess oxygen is about 2-4% as a target to maximize efficiency and minimize formation of NOx, while still ensuring complete combustion of fuel. However, effectively and accurately monitoring airflow to maintain this small excess margin in industrial combustion systems is more complicated than monitoring the fuel flow.

For better boiler control, maintaining a balanced fuel and air mixture can be accomplished in a couple of different ways.

Parallel Positioning (PP) Systems

In a parallel positioning system, the objective is to link the position of the air damper with the position of the gas valve to maintain an acceptable air-to-fuel ratio that is based on a process setpoint, such as a required steam pressure.

In this type of cross-limiting system, the damper/valve controls are physically linked, so it is not possible to control fuel flow and airflow individually. PP systems may work reasonably well initially but will drift away from the desired fuel-air ratio over time as valve seals and damper linkages wear. 

Additionally, a PP system will not respond to fluctuations in process temperature, duct static pressure, and barometric pressure changes, which alter air density and affect the air-fuel ratio.

Adding O2 Trim to PP Systems

A second option for airflow control is known as parallel positioning with O2 trim. In this control scheme, O2 trim systems are used to continuously monitor the amount of oxygen in the exhaust gas and provide feedback to an automatic positioner for the air damper. 

The objective is to “trim” the airflow to drive excess oxygen lower (closer to 2-4% excess O2). PP with O2 trim is the most common type of control that is applied to combustion systems across many industries.

Fully Metered Combustion Air Control Systems

In this scheme, flow measurement devices are added for both fuel and air flow measurement to provide instant feedback for maintaining the air-to-fuel ratio at its most efficient point. In a fully metered system, accurate mass-based airflow measurement is crucial.

The rationale behind fully metered systems is to address multiple issues simultaneously: promote efficient fuel use, control emissions, extend burner lifetime (especially with ultralow-NOx burners), and expand the efficient operating range. Fully metered systems typically include O2 trim, as well. This provides the ultimate in control and final operation.

Advanced Control Technology for Increased Efficiency

The fully metered combustion control system is only as good as the measurement technologies used as inputs. These can include flow conditioners followed by multipoint Pitot averaging arrays that are used to accurately sense the stratified velocity profile.

Next, a low-range multivariable transmitter is used to amplify the differential pressure and compensate for changes in atmospheric pressure, static pressure, and process temperature. This provides a 4–20mA output with a linear relationship to mass flow to the fully metered control system.

In cases where particulates are present, an automatic purge system also is integrated into the flow measurement system, ensuring that the measurement is never lost due to the sensing ports plugging. During this purge cycle, the signals are held at their last values.

Benefits of Fully Metered Combustion Control Systems

The degree of actual control enabled with a PP/O2 trim system is not very large. In contrast, state-of-the-art fully metered systems have the best ability to optimize the efficiency of burners and can address multiple challenges at once, including efficiency, maintenance, safety, and emissions.

Fully metered systems allow tighter control of the air-fuel ratio throughout the full operating range of the boiler and ensure that emissions are within specifications. They also allow a much higher degree of control for operations over tuning the system, so operators can closely match the optimal air-fuel ratio even throughout process temperature swings, barometric pressure, and static pressure changes.

The greater degree of combustion air control means there are fewer boiler trips, so less downtime across full operating ranges of the combustion burner when the air-fuel ratio gets out of balance. Fully metered systems also can easily adapt to fuel type or composition changes such as switching between natural gas and fuel oil or changing natural gas suppliers.
The challenge of realizing fuel savings while maintaining the ideal air-to-fuel ratio for process controls is felt by plant managers everywhere. Discover more information about Air Monitor’s combustion air flow measurement technologies and how they can benefit your industrial boiler application by clicking below.

Efficient Air-to-Fuel Ratio

In chemistry, stoichiometry is the method for balancing chemical equations to calculate the exact amount of individual reactants needed to ensure that all the reactants are used up with no excess left over from the reaction. For example, a simple combustion equation for burning the methane (CH₄) in natural gas is:

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

According to this stoichiometric equation, a perfectly balanced combustion process would require exactly two oxygen (O₂) molecules to burn each methane molecule. The reaction would produce one molecule of carbon dioxide (CO₂) and two molecules of water (H₂O) along with excess heat to be used for the process.
If the equation is not perfectly balanced, for example, if there is not enough O₂  to completely combust or burn each CH₄ molecule, the reaction will produce some carbon monoxide (CO) instead of CO₂.

First, by requiring the system to heat a larger volume of gas than is needed decreases the ability for the system to transfer heat from the flame to the steam (in the case of a boiler) or to the process fluid.

Second, too much excess oxygen will force the fan to operate at elevated speeds, which wastes energy over time and increases emissions. A common margin of excess air is about 2-4% more air than fuel. 

This target maximizes efficiency and minimizes formation of NOx—a pollutant generated from nitrogen in the air reacting with oxygen—while still ensuring the complete combustion of fuel.

3 Methods for Maintaining an Efficient Air-to-Fuel Ratio

To accurately maintain this small excess margin in industrial combustion systems, combustion control systems need accurate airflow measurement and control in addition to fuel flow monitoring. In general, operators use one of three different methods to monitor and maintain the proper amount of air and fuel in an industrial plant:

• Parallel positioning (PP)
• PP with O₂ trim
• Fully metered airflow control systems

Parallel positioning systems are relatively straightforward technically and are generally the least expensive option for controlling airflow. These provide some degree of control for stable loads and consistent burner function and offer the lowest capital cost of the three options. 

However, PP is not effective for variable loads or rapid swings and is the least able to maintain optimal efficiency.

PP with O₂ trim systems continuously monitor the amount of oxygen in the exhaust gas and provide feedback to an automatic positioner for the air damper. The objective is to “trim” the airflow in order to drive excess oxygen lower (closer to 2-4% excess O₂).

PP with O₂ trim is the most common type of control that is applied to combustion systems across a number of industries. This type of system, however, is best applied to boilers at steady-state operation, while real-world boilers run at constantly varying steam loads. The degree of actual control enabled with a PP/O₂ trim system is not very large and this system is not the best choice for boilers with broad load ranges.

Fully metered airflow control systems

Minimizing the amount of excess air without creating a fuel-rich environment can best be achieved with the help of fully metered systems that effectively monitor and control airflow. These systems have the best ability to optimize the efficiency of burners and can address multiple challenges at once including efficiency, maintenance, safety, and emissions. 

They allow tighter control of air-to-fuel ratio throughout the full operating range of the boiler and ensure that emissions are within specifications.

Fully metered systems allow a much higher degree of control for operations over tuning the system using PP with O₂ trim. With a fully metered system, operators can closely match the optimal air-fuel ratio even throughout process temperature swings, barometric pressure, and static pressure changes. 

The greater degree of control means there are fewer boiler trips and therefore less downtime across full operating ranges of the combustion burner when the air-to-fuel ratio gets out of balance. Fully metered systems also can easily adapt to fuel type or composition changes such as switching between natural gas and fuel oil or changing natural gas suppliers.

Because a poor ratio of air to fuel can contribute to failures of burner internals as well as soot buildup, there is a connection to unplanned shutdowns. Better airflow control enabled by fully metered systems results in fewer chronic maintenance issues, such as burner replacements.

For large-sized boilers, good air monitoring allows the possibility of using a predictive emissions control system (PEMS) versus continuous (CEMS) to report emissions to air districts or the EPA. 

A PEMS system is one-third the cost of a CEMS but requires that plant operators can maintain high levels of confidence in the control system for airflow to ensure the measurement is accurate.

A Fully Metered Airflow Control System Can Benefit Your Industrial Application

The benefits of a fully metered airflow control system come with costs and added complexity— however, the benefits far outweigh the costs. The cost of this type of system varies but is slightly more than that of a PP with O2 trim.
The challenge of maximizing fuel economy using the stoichiometric air-to-fuel ratio is felt by plant managers everywhere. With a fully metered combustion airflow measurement system, operators can expect to see measurable efficiency improvements and emissions reductions that will increase their bottom line.

Of all the things hitting your desk, why should you worry about space pressure control in your facility? We’ve done the research for you and have identified three major benefits.

Top 3 Industrial Benefits of Facility and Space Pressure Control

#1 Quality production output/improved process control:

Where purity standards must be maintained for product quality, production output, and where improved process control is required, facility and space pressure control become important. Space pressure control can help avoid contamination of clean areas by dust, pollutants, or other undesired conditions that might impact the quality of your product.

Here’s a list of examples:

• Manufacturing pharmaceuticals, electronics, food & beverage
• Facilities with “clean rooms”
• Furnace pressure control for battery recyclers (EPA regulation tied to this (40 CFR Part 63 National Emissions Standards for Hazardous Air Pollutants (NESHAP) from Secondary Lead Smelting)) for optimal process control and quality production output

#2 Cost Savings:

Without pressure control, there can be some unintended negative consequences to day-to-day functions that end up increasing facility maintenance costs. Correcting these issues help minimize system cycling and excess power use for energy savings. Let’s look at a couple examples.

First, pressure sensing devices must be used to detect pressure imbalances. Without accurate pressure sensing devices in key areas, the control system cannot function efficiently. When a pressure imbalance occurs causing exfiltration or infiltration, the control system will continue to cycle attempting to restore an unattainable balance and energy costs will soar.

Next, have you ever had trouble opening a door because of pressure imbalances? This isn’t just an inconvenience for workers trying to easily navigate the facility, sometimes with product, inventory, or other gear. It can also be a huge energy sink! Sometimes doors will not fully shut from pressure imbalances, too. This causes an air stream to form that allows heated/cooled facility air to escape the space resulting in higher energy costs. A good pressure system with accurate pressure sensors can correct these issues.

Lastly, pressure control can have some long-term benefits – like detecting leaks. The analysis of long-term aggregate pressure level data can pinpoint troublesome areas. Further inspection of the problem areas will uncover any leaks in the airflow system. Repaired leaks will create a more efficient system and save energy costs over time.

#3 Worker safety in industrial facilities:

Maintaining duct pressure levels relative to room or area pressure levels is key to worker safety. This is done by measuring pressure in duct, measuring in the space, then comparing the duct pressure to ambient air pressure. The control system adjusts – based on this data – to balance pressure accordingly. Let’s look at two examples:

• Avoid the introduction of contaminants into work areas that jeopardize worker health and safety 
• Space pressure control around smelting areas to protect workers in a smelting facility or foundry

Get up to date on the regulations that might be impacting your facility safety requirements.

How Does it Work?

To get a better picture, let’s look at how a theoretical example of a facility pressure control system would work.

First, the control system may measure the outdoor pressure, which changes periodically due to changes in weather. Then, the control system may monitor the pressure of spaces inside the facility. Some systems incorporate airflow measurement into this control scheme. Pressure data feeds into the control system and triggers the airflow system to open and close dampeners allowing airflow to key areas in the facility. While the air flows through the system, duct static pressure sensors feed data to the control system. Duct pressure data and ambient pressure data are compared with the pressure settings that the operator has determined are ideal atmospheric environments for the work or processes occurring in that space of the facility. In some cases, the control system may be set up to maintain a positive/negative pressure relationship between two areas to avoid contamination of production output.

Why it’s Important

Manufacturing facilities must be able to meet purity standards when producing medicines, food, beverages, electronics, or other dust- and contaminant-sensitive products. Clean rooms create a safe environment, free of contaminants, where these products can be manufactured to the high-quality standards set by the FDA or other regulatory bodies.

Industrial facilities that operate with or produce harmful pollutants like volatile organic compounds (VOCs) or hazardous air pollutants (HAPs) must be able to maintain safe working environments for their employees and ensure that these pollutants are being destroyed per regulations in air waste processing equipment like thermal oxidizers and not venting into work areas or the atmosphere.

• Long term pressure monitoring of the waste airflow ducts compared to the ambient pressure can help detect leaks in the waste airflow feeds.
• Keep harmful gases out of smelting and combustion processes

Although OSHA does not have specific requirements for workplace indoor air quality (IAQ), they do require employers to provide their workers with a safe workplace that does not have any known hazards that cause or are likely to cause death or serious injury. Some states, like California and New Jersey, have their own indoor air regulations.

What Can you Do to Manage Industrial Facility Pressure Challenges?

Ok. So, now that we all understand the benefits of having a pressure system and how it works, let’s discuss the challenges that commonly arise when trying to set up a pressure system and the solutions that Air Monitor offers.

Challenge 1: Design challenges

Air Monitor Solution: Full start-up services, duct traverse studies & training

Air Monitor has over 50 years’ experience providing premier airflow measurement and pressure measurement systems to industrial process markets. Our technical team ensures that your system will meet the standards that your challenging application requires.

Challenge 2: Accuracy

Air Monitor Solution: Excellent stability, repeatability, and accuracy

Air Monitor products work together with the best-in-class accuracy of our transmitters to keep your control system running smoothly.

Challenge 3: Tough industrial settings

Air Monitor Solution: Range of Offerings and Materials for Tough Environments

Air Monitor’s rugged product line can operate in corrosive, high temperature, and particulate-laden air streams.

Look at our line of products tailored to meet your facility space pressure control needs:

• S.A.P. (Static Pressure Ports)
  • These sensors detect the pressure of an indoor space
• S.O.A.P. (Static Outdoor Air Ports)
  • These sensors detect the pressure outdoors of a facility
• STAT-probe
  • These sensors detect the pressure inside of a duct
• VELTRON II Transmitter,
  • used in conjunction with our pressure sensors and airflow measurement stations can measure and help regulate space/room/area pressure
• VEL-trol II Transmitter
  • Same great features as the VELTRON II, also includes Proportional Integral Derivative (PID) control

Feeling under pressure? Talk to an expert at Air Monitor about what you’re experiencing at your facility: Contact Us